1. Field of the Invention
The invention relates to touch panels, more particularly, to a touch detecting device for touch panels.
2. Description of the Related Art
FIG. 1A shows a schematic diagram of a conventional sensor array. Referring to FIG. 1A, a conventional touch panel uses a sensor array 100 to detect a location and a pressure of either a user's finger or a stylus. The sensor array 100 includes two layers of conductors. A first group of long strips of conductors on x-axis is arranged orthogonally to a second group of long strips of conductors on y-axis. Depending on the touch panel being capacitive or resistive, a corresponding sensing element, e.g., a resistor or a capacitor, is installed at each intersection of two strips of conductors as shown in FIGS. 1B and 1C. If the finger or the stylus touches the sensor array 100 at a certain position, its corresponding capacitance or resistance will change. Assuming that one specific row of the sensor array 100 is enabled, after an input signal is applied to the specific row, six output signal values (0, 1, 2, 3, 4, 3) are respectively measured at six different nodes of the specific row, thus indicating different touch pressure. In this case, the fifth node has a relatively high pressure. If each row and column of the sensor array 100 is sequentially or simultaneously measured, a two-dimensional map of touch pressures on the sensor array 100 will be obtained, indicating the pressure magnitudes and the positions of multiple finger touches. However, the above sensor array 100 needs additional capacitor or resistor manufacturing process and thus the hardware cost is increased.
On the other hand, the sensor array 100 is subject to noise interference, so it is difficult for the system to distinguish the finger touch from noise interference if there are variations of resistance or capacitance. For example, with respect to a capacitor having a capacitance of 50 pF, a variation of capacitance caused by the finger touch is about 1 pF. When the capacitor is charged to a voltage level of 2V, a variation of resistance caused by the finger touch is about 40 mV while the voltage level of noise interference is about several tens of mV. Accordingly, its corresponding signal-to-noise (SNR) value is not large enough, so it is easy to cause the system to misjudge.
In particular, most of current touch panels are manipulated in a complicated manner and loaded with several wireless communication functions (e.g., infrared or Bluetooth connectivity) or a backlight plate. Accordingly, noise sources of a touch panel are various, including 1/f noise, white Gaussian white noise, power noise, 50/60 Hz noise, communication microwave noise and backlight noise. Reference is now made to FIG. 2, which is a schematic diagram illustrating a common noise distribution in frequency domain. In general, lowpass filters are configured to filter out high frequency noise. With respect to low frequency noise (e.g., 1/f noise and 50/60 Hz noise), when a lowpass filter in use is designed to have operate at a lower cutoff frequency, lower frequency noise can be eliminated, yet its response time is also increased as well. For example, assuming that a lowpass filter in use is designed to operate at a cutoff frequency of 10 Hz to eliminate 60 Hz noise, its response time will be delayed by 0.1 second, equivalent to 0.1 second delay in drawing lines. When there are delays between finger touches and the responses of applications, it will cause inconvenience for users.